Purification of Diatomaceous Earth using Acid Leaching Process to Produce High Purity Silica for Solar-Grade Silicon

Recently, the demand for solar-grade silicon (SOG-Si) with 6-7N (99.9999~9%) level of purity is dramatically increasing due to its popularity of photovoltaic power generation. The conventional process, Siemens method, is majorly used for SOG-Si production, which, however, consumes huge amount of energy and time. So, there is a need to develop an alternate production process for SOG-Si. As the cost-effective process, we are now focusing on a wet chemical process for purification of silica, followed by electrochemical reduction of silica to SOG-Si [1]. This purification process consists of two main steps: (i) acid leaching including dissolution & precipitation treatment of diatomaceous earth as an abundant Si source and (ii) liquid-liquid extraction for precise elimination of lighter element impurities that are possible to influence the semiconductor property of Si. Aim of the present work is to increase the extraction efficiency of each process to be optimized and combining them to complete the whole process. First, we optimized the acid leaching process. This process performs pH adjustments of strong alkaline solution with diatomaceous earth dissolved to separate impurity-rich phase and silica. It has been shown that more than 90% of the heavy element (Fe, Al, etc.) impurities precipitate by decreasing pH [2]. However, since NaOH was used in the preparation of initial alkaline solution, much amount of Na impurities turned out to be included in the final silica product. For this, we employed the mixture of NaOH and Tetramethylammonium hydroxide (TMAH) to lower the concentration of Na in the alkaline solution preparation. Experimental results suggested that some amount of Na ion was necessary to be included in the solution in order to provide sufficient dissolution rate of silica. This led us to the optimal concentration of NaOH and TMAH in the mixture: 0.05 M NaOH and 1.40 M TMAH. Further purification, experiment results concluded that this mixture effectively prevented the inclusion of Na impurity, resulting in 5N level of purity when the process was repeated 3 times. In spite of the achievement shown above, the lighter element, in particular, B is hard to be eliminated from the leaching process, because the pH dependence of solubility of B is similar to that of Si. To further eliminate this impurity, the liquid-liquid extraction process is employed. Since our previous results indicated that the effective mixing of aqueous and organic phase leads to the best efficiency of B elimination, we first designed flow type reactor with 3D printing technology. The reactor was designed to have 2 characteristic structures: a throat like structure that shows gradually diverging part after converging (converging/diverging structure) near Y shaped inlets and obstacles structure on downstream [3]. The high-speed camera demonstrated the reactor offered a fully dispersed flow regime, which leaded to the efficient solution mixing. Based upon these results, we carried out a model experiment, using silica powder of 99.9 % purity with trace amount of B was dissolved in NaOH solution (aqueous phase) and injected into one of the inlet of the liquid-liquid extraction reactor. The extractant, 2,2,4-trimethyl-1,3-pentanediol (TMPD) solution (organic phase) was injected into the other inlet. The extraction was repeated 5 times at 380 ml/min and the efficiency was found to be 99.7% leaving less than 0.1 ppm of B in silica (7N purity). From the above results, it is concluded that we have achieved efficient process to prepare 7N level silicon, by combining optimized leaching and liquid-liquid extraction with designed reactor. This could be a promising method to produce SOG-Si with lower cost than the present process at industries.

Chemically Amplified Photosensitive Polybenzoxazoles Based on <i>tert</i>-Butoxycarbonyl Protected Hyperbranched Poly(<i>o</i>-hydroxyamide)s

With the sharpening of optical images, the capability of resist materials has become a serious concern in lithography. The dissolution of a resist polymer is key to the realization of ultrafine patterning. However, the details of the dissolution of resist polymers remain unclarified. In this study, the relationships of surface free energy with swelling and dissolution kinetics were investigated using poly(4-hydroxystyrene) (PHS) film with triphenylsulfonium-nonaflate (TPS-nf). Developers were water and 2.38 wt% tetramethylammonium hydroxide (TMAH) aqueous solution. PHS and TPS-nf are a typical backbone polymer (a dissolution agent) and a typical acid generator of chemically amplified resists, respectively. The water intake and dissolution of PHS film with TPS-nf became fast with increasing UV exposure dose. It was found that the increase in the polar components (particularly, the hydrogen bonding component) and the decrease in the dispersion component of surface free energy underlie the fast water intake and dissolution.

FCC PtRu nanoparticles with narrow size distribution were prepared by polyalcohol reduction of platinum acetylacetonate and ruthenium acetylacetonate in diphenyl ether, using oleylamine as the capping agent. The particle size ranged from 3.5 to 6.5 nm and was controlled by varying the amount of capping agent added in the synthesis. Adjusting the stoichiometric ratio of introduced Pt and Ru precursors yielded particles with different compositions. A simple fractionation was employed to narrow the size distribution of particles and nearly monodispersed particles were obtained. To activate the catalytic activity of the particles, oleylamine bound to the particle surface was removed through repeated washing of the as-prepared particles with ethanol and tetramethylammonium hydroxide (TMAOH) aqueous solution. The washed particles could be well redispersed and electrostatically stabilized in TMAOH aqueous solution and uniformly loaded on a Vulcan XC-72 carbon support. A cyclic voltammetry (CV) study revealed that the carbon-supported, cleaned PtRu nanoparticles exhibit considerably higher electrocatalytic activity for methanol oxidation compared with the as-prepared particles that have oleylamine ligands.

This study is a continuation of long-term work to build a quantitative model of genesis of deposits of Ta, Nb and Li, connected with granites. Importance of the problem is the need to use the available experimental data to estimate quantitatively a possible role of hydrothermal transport and redeposition of tantalum and niobium under physical-chemical conditions typical for Ta and Nb deposits, generally connected with alkaline rocks. Earlier we investigated concentration and temperature dependence of the tantalum and niobium oxides solubility in the sodium alkaline and carbonate solutions in a wide range of temperature, compositions and concentrations of solutions at P=1000 bar [Kotova and Zaraisky, 2009; Zaraisky et al., 2009]. The choice of sodium specificity of the solutions is related to the fact, that Ta-Nb mineralization in lithium-fluoric granites (“apogranites”), alkaline granites, alkaline metasomatites, sienites and carbonates is closely associated with sodium metasomatism (albitization, ribektization, egirinization). In order to practically cover the entire range of conditions for the formation of Ta-Nb deposits the experimental studies of Та2О5 and Nb2O5 solubility were carried out in aqueous alkaline solutions at P=500 bar. The tantalum and niobium oxides- chemical reagents Та2О5 and Nb2O5, being the analogues of tantite and nioboxide, seldom occurring in nature, were used as initial material in the runs. The concentration dependence of the Та2О5 and Nb2O5 solubility at T=550 o C, P=500 bar in the solutions of Na2CO3 and NaOH with the concentration (0.01; 0.1; 0.5, 1.0 и 2.0 m) in the presence of oxygen buffer Co–CoO was studied. Run duration was 21 days. Experiments were performed on a hydrothermal line. The capsules and the container with buffer were sealed into cold-seal pressure vessel of Tuttle type with big working volume that gives possibility to isolate the capsules from the container with buffer. The same technique was used to study Та2О5 solubility in fluoride and chloride solutions. After the run, the quenched aqueous solutions and the solid products were separated using a centrifuge. The solutions were then analyzed using ICP/MS (Inductively Coupled Plasma Mass Spectrometry) and ICP/AES (Atomic Emission Spectroscopy) for Nb, Ta, Mn, and Fe and admixture elements Ti, W, and Sn,. The composition of the solid product was characterized using optical microscopy, X-ray diffraction, and electron microprobe analysis (Cam Scan MV 2300 (VEGA TS5130MM). Experimental data are shown in fig. 1, 2, 3 and 4. The Ta2O5 solubility isotherms was found to have a negative trend in all studied alkaline and carbonate solutions at P=1000 bar. The concentration dependence of the Та2О5 was found to be more complicated at P=500 bar. In NaOH solutions a pressure decrease from 1000 bar to 500 leads to a decrease of tantalum content by a factor of 1 between 0.01 and 0.05 m NaOH. However, the Ta2O5 solubility increases by a factor of 1 in aqueous solutions with NaOH concentrations of 1 m and greater (fig. 1). In the Na2CO3 solutions with decreasing pressure from 1000 bar to 500 the tantalum content decreases by a factor of 1 (fig. 2). Experimentally established fact of the weak solubility of tantalum oxide in aqueous alkaline and carbonate solutions is fundamentally important for understanding of genesis of Ta deposits. The experimental data show that, for the range of concentrations, temperatures and pressures considered in this study, the Ta2O5 solubility is very low with concentrations on the order of 10 -5 to 10 -7 m in aqueous alkaline and carbonate solutions. The investigations have shown that at such low concentrations it is difficult to speak about possibility of Ta hydrothermal transport by alkaline and carbonate solutions. In NaOH and Na2CO3 solutions oxide tantalum has incongruent solubility. As solid phases wonderfully cut crystals of Na-tantite Na2Ta4O11 with the hexaoctahedric form typical for pyrochloride are formed in 0.01 and 0.1m NaOH and

In recent years, photovoltaic power generation using silicon solar cells has been increasing significantly. Currently, the solar-grade silicon (SOG-Si) with 6-7N (99.9999~9%) level of purity, for photovoltaic power generation is manufactured using Siemens process, which consumes drastic amount of energy & time, and uses “silica ore” as raw material that might be possibly depleted soon. There are number of alternative approaches that have been developed for solar-grade silicon such as the metallurgical process, zinc reduction process or hydrogenation process. However, these processes still require substantial energy input for the fusion or gasification and reduction of metallurgical-grade silicon (MG-Si). Therefore, there is a need to develop an alternate production process for SOG-Si that would stabilize the problems including the cost factor. Our research group focuses on the development on new preparation process of SOG-Si from diatomaceous earth as that is one of the promising candidate with abundant raw material of silica reserves: combination of wet chemical process and channel reactor process. The wet chemical process includes dissolution/precipitation treatment of diatomaceous earth with NaOH + TMAH mixture solvent with pH adjustment, followed by acid leaching [1-2]. The dissolution/precipitation treatment preliminarily purifies silica by utilizing difference in pH dependences of solubility of Si and other impurities. In the acid leaching, HCl aqueous solution subsequently removes impurities from gel-state silica. These processes are profitable to remove large amount of impurities, which were however found to be hard to eliminate B that affects significantly the electrical properties of SOG-Si. Series of our studies formerly found the followings: (i) liquid-liquid extraction process using 2,2,4-trimethyl-1,3-pentanediol (TMPD) as an extractant in organic phase (toluene) works very well to extract the B (boric acid in solution) from aqueous solution, (ii) flow type reactors are more suitable and profitable than batch type reactors for practical uses as it provides a long contact period [3-4], and (iii) the liquid-liquid interfacial reaction rate of boric acid and TMPD is sufficiently high to set the process diffusion-limited. Based on these findings, this study focuses on the design and testing of the practical channel rector in a larger scale (135x100x10 mm), which is fabricated using 3D printing technology. The channel was designed to have 2 characteristic structures: a throat structure that shows gradually diverging part after converging (converging/diverging structure) near Y shaped inlets and obstacles structure on downstream, as shown in Fig.1. The silica solution (aqueous phase) was prepared and injected into one of the inlet and TMPD (organic phase) solution into the other inlet. Vortices were generated around diverging part, leading to efficient mixing of each phase to produce smaller droplets of one phase dispersed into the other, which was retained by obstacles part efficiently. This continuous droplet production enhances mass transfer of boric acid to the interface to react with TMPD. 5 times repeating application found to be 99.65% removal of B at the flow rate 350 ml/min. From these results, this kind of converging/diverging structure and presence of obstacles will enhance the mixing efficiency between aqueous and organic phases, which in turn produces maximum extraction efficiency. These kinds of channel reactors are profitable for silica purification at industries.

We present herein a simple protocol of growing a patterned ZnO nanowire by etching of ZnO seed layer in the tetramethyl ammonium hydroxide (TMAH) solution. The ZnO seed layer was fabricated by sol–gel method using zinc acetate solution and patterned by using photolithographic method. Patterned ZnO seed layer as etched in the TMAH solution, followed by growth of ZnO nanowires by hydrothermal method. Remarkable point of present ZnO seed layer patterning is that development of UV-exposed photoresist and etching of ZnO seed layer is subsequently processed in aqueous TMAH solution without interruption. The grown ZnO nanowires were analyzed using XRD patterns to exhibit high purity and degree of crystallinity, and showed very good pattern fidelity.

Cotton fabric was treated with aqueous solutions of tetramethylammonium hydroxide (TMAH) and sodium hydroxide of various concentrations. The sorption of both chemicals, the specific weight of the fabric, the cellulose I – cellulose II transition, the BET surface area, the iodine sorption capacity, and the water retention value were measured and compared. Although less TMAH was adsorbed by cellulose than NaOH, the TMAH proved to be a more effective swelling agent for cellulose. The difference between their swelling effect starts to be significant at a concentration of about 2 mol·dm–3.

Cotton fabric was treated with aqueous solutions of tetramethylammonium hydroxide (TMAH) and sodium hydroxide of various concentrations. The sorption of both chemicals, the specific weight of the fabric, the cellulose I – cellulose II transition, the BET surface area, the iodine sorption capacity, and the water retention value were measured and compared. Although less TMAH was adsorbed by cellulose than NaOH, the TMAH proved to be a more effective swelling agent for cellulose. The difference between their swelling effect starts to be significant at a concentration of about 2 mol·dm–3.

BACKGROUNDLignocellulose is usually pretreated as a raw material, and then it undergoes hydrolysis, fermentation, and other processes to produce biofuels, which is conducive to improving the utilization of agricultural and forestry wastes. However, most pretreatments, such as acid–base, CO2 explosion, and microbial degradation, could enhance the yield of enzymatically hydrolyzed (EH) sugars, but the conditions are relatively harsh, environmentally unfriendly, and economically unfeasible. Ionic liquids, in view of their excellent physical and chemical properties and strong solubility, have been proven to be an effective green solvent for pretreating lignocellulose. The aim of this study was to exploit tetrabutylammonium hydroxide (TBAOH) and tetramethylammonium hydroxide (TMAOH) aqueous solutions to pretreat corn cob under relatively mild conditions.RESULTSThe optimal pretreatment conditions for the two solutions were 30 °C, TBAOH concentration of 20 wt%, 60 min, 5 wt/wt, and 20 °C, TMAOH concentration of 10 wt%, 60 min, 5 wt/wt, and the corresponding sugar yields were 88.9% and 92.2%, respectively. Compared with the unpretreated corn cob, the yield of EH sugar increased by about 2.7–2.8 times. Characterization results confirmed that hemicelluloses and lignin were removed from the raw material during the pretreatment process, and that the cellulose was purified. Furthermore, after 5 cycles of reuse of the TBAOH and TMAOH aqueous solutions, the pretreated corn cob could still achieve a sugar yield higher than 70%.CONCLUSIONThe pretreatment with TBAOH and TMAOH aqueous solutions significantly enhanced the enzymatic hydrolysis, which provided a green, simple, and economical new approach for the pretreatment of agricultural waste biomass. © 2021 Society of Chemical Industry

We combine spectroscopic ellipsometry (SE), Fourier transform infrared spectroscopy (FT-IR), kinetic Monte Carlo simulations (KMC) and convex corner undercutting analysis in order to characterize and explain the effect of the addition of small amounts of surfactant in alkaline aqueous solutions, such as Triton X-100 in tetra methyl ammonium hydroxide (TMAH). We propose that the surfactant is adsorbed at the silicon–etchant interface as a thin layer, acting as a filter that moderates the surface reactivity by reducing the amount of reactant molecules that reach the surface. According to the SE and FT-IR measurements, the thickness of the adsorbed layer is an orientation- and concentration-dependent quantity, mostly due to the orientation dependence of the surface density of H-terminations and the concentration dependence of the relative rates of the underlying oxidation and etching reactions, which have a direct impact on the number of OH terminations. For partial OH coverage of the surface, the hydration of the OH group effectively acts as an anchoring location for the hydration shell of a surfactant molecule, thus enabling the formation of hydration bridges that amplify the adsorption density of the surfactant. At high concentration, the model explains the large reduction in the etch rate of the exact and vicinal Si{1 1 0} surfaces, and the small changes in the etch rates for the exact and vicinal Si{1 0 0} surfaces. At low concentration, it explains how the etch rate for both families is significantly reduced. The orientation and concentration dependence of the surfactant adsorption explains the dramatic differences in the micron-scale wet-etched patterns obtained using TMAH and TMAH+Triton for microelectromechanical systems applications.

Photoresists have been widely used as patterning materials for electric devices such as displays and semiconductor. Understanding pattern formation mechanism is essential for the efficient development of resist materials. In this study, we investigated the dissolution kinetics of poly(4-hydroxystyrene) (PHS) with weight-average molecular weights (Mw) of 9000-30000 and molecular weight distribution (Mw/Mn) of 1.07-1.20. The dissolution kinetics of PHS films was observed in tetramethylammonium hydroxide (TMAH) aqueous developers using a quartz crystal microbalance (QCM) method. The TMAH concentration was changed from 0 to 2.38 wt%. The obtained data were analyzed using polynomial regression to clarify the effects of Mw and Mw/Mn on the dissolution kinetics of PHS films. From the results of analysis, both dissolving and swelling behavior largely depended on Mw/Mn. Mw had a little effect on the dissolving, and however, had a large effect on the swelling in dilute TMAH aqueous solution.

Lactones are almost ubiquitously employed in 193 nm resists to increase the polarity of hydrophobic alicyclic polymers. What else do lactones do in 193 nm resists? We studied the behavior of methacrylate (MA) resists consisting of different protecting groups, hexafluoroalcohols, and norbornane lactone methacrylate (NLM, 2-oxo-3-oxatricyclo[4.2.1.04,8]nonan-5-yl methacrylate). When the protecting group is large [ethylcyclooctyl (ECO) and methyladamantyl (MAd)], thinning of the resist film that occurs in highly exposed areas upon postexposure bake (PEB) is significantly smaller than what is expected from the polymer composition. When the concentration of isopropylhexafluoroalcohol methacrylate (iPrHFAMA) is increased in the ECOMA-NLM polymer, the thinning increases and reaches 100% of theory and the ECOMA-norbornenehexafluoroalcohol methacrylate (NBHFAMA) resist loses quantitative thickness in highly-exposed areas upon PEB at 90 &deg;C. This indicates that small lactones which are more basic than esters can trap deprotection fragments especially when the protecting group is large. Such entrapment was detected by IR spectroscopy and also observed at temperatures as high as 200 &deg;C in thermogravimetric analysis (TGA). Incorporation of lactone appears to decrease the bake temperature sensitivity and the sensitivity of the resist perhaps due to trapping of photochemically generated acids by basic lactone. The lactone ring can be hydrolyzed during aqueous base development but does not seem to affect the dissolution rate, indicating that hydrolysis occurs in aqueous base solution after dissolution. Poly(methacrylic acid-NLM) dissolves as fast as poly(methacrylic acid) in 0.26 N tetramethylammonium hydroxide (TMAH) aqueous solution. While exposed P(ECOMA₄₇-NLM₅₃) resist dissolves in 0.26 N developer at about the same rate as authentically prepared poly(methacrylic acid₄₇-NLM₅₃), the dissolution rate of highly-exposed P(MAdMA₄₄-NLM₅₆) resist is much slower, indicating that the deprotection fragment from the former does not interfere with the development but that from the latter does. When the NLM concentration is increased to 75 %, highly exposed P(ECOMA-NLM) resist dissolves slowly at ca. 600 A/sec and swells significantly, indicating that NLM can be a dissolution inhibitor and swelling enhancer when its concentration is high. Low activation energy protecting groups such as ethylcyclooctyl allows imaging at temperatures as low as 60 &deg;C. However, the temperature dependence of the dose to clear is very large and the chemical contrast is quite small in the low temperature range. Thus, for PEB temperature stability and contrast enhancement, baking 20-30 &deg;C above the lowest practical temperature is recommended.

The hydrolysis of sodium trimeta- and tetrametaphosphates was run in water, dioxane–water, and formic acid–water solvents with an initial concentration of 0.025 mol/l at various pH values and temperatures. The hydrolysis of both the metaphosphates followed first-order kinetics with respect to the concentration of the phosphate under all the conditions studied and was an acid- and base-catalyzed reaction. In acidic solutions, the rate of hydrolysis of both the metaphosphates in water is faster than that in formic acid–water and slower than that in dioxane-water. Therefore, it is concluded that, in the formic acid–water solvent, the nucleophilicity of the water molecule may decrease on account of the solvation of the water molecule with the acid, while, in the dioxane–water solvent, since the scission of the hydrogen bond between water molecules may occur by the interaction of dioxane and water molecules, the nucleophilicity of the water molecule may increase. The hydrolysis of both metaphosphates in acidic solutions is considered to be an SN2 reaction, because the overall-reaction rate is highly dependent upon the nucleophilicity of the water molecule. In basic solutions, the rate of hydrolysis of sodium trimeta- and tetrametaphosphates in water is faster than or almost the same as that in dioxane-water. Consequently, the mechanism of the hydrolysis of both the metaphosphates in basic solutions seems to differ from that in acidic solutions. The increase in the rate of hydrolysis of small-ring phosphates in acidic solutions depends upon the activation energy and frequency factor, while that in basic solutions depends significantly upon the frequency factor. This may cause the difference in the solvent effect on the rate of hydrolysis of small-ring phosphates between acidic and basic solutions. The activation energy of the hydrolysis of both ring phosphates was 20–40 kcal/mol in the pH range of 1.0–12.5. The rate of hydrolysis of the metaphosphates in an aqueous sodium hydroxide solution is faster than that in an aqueous tetramethyl ammonium hydroxide solution and this can be attributed to catalysis by a sodium ion. The hydrolysis of sodium tripolyphosphate was also carried out in basic water and dioxane–water solvents. The solvent effect on the rate of hydrolysis of tripolyphosphate in basic solutions was the same as that in acidic solutions.

Kinetic study of silica dissolution in aqueous solutions of aromatic organic electrolytes